Messages in this thread | | | Date | Mon, 29 Nov 2021 23:12:41 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults |
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On Mon, Nov 29, 2021 at 01:53:01PM -0800, Linus Torvalds wrote: > On Mon, Nov 29, 2021 at 12:56 PM Catalin Marinas > <catalin.marinas@arm.com> wrote: > > I think that would be useful, though it doesn't solve the potential > > livelock with sub-page faults. > > I was assuming we'd just do the sub-page faults. > > In fact, I was assuming we'd basically just replace all the PAGE_ALIGN > and PAGE_SIZE with SUBPAGE_{ALIGN,SIZE}, together with something like > > if (size > PAGE_SIZE) > size = PAGE_SIZE; > > to limit that size thing (or possibly make that "min size" be a > parameter, so that people who have things like that "I need at least > this initial structure to be copied" issue can document their minimum > size needs).
Ah, so fault_in_writeable() would never fault in the whole range (if too large). When copy_to_user() goes beyond the faulted in range, it may fail and we go back to fault in a bit more of the range. A copy loop would be equivalent to:
fault_addr = ubuf; end = ubuf + size; while (1) { if (fault_in_writeable(fault_addr, min(PAGE_SIZE, end - fault_addr))) break; left = copy_to_user(ubuf, kbuf, size); if (!left) break; fault_addr = end - left; }
That should work. I'll think about it tomorrow, getting late over here.
(I may still keep the sub-page probing in the arch code, see my earlier exchanges with Andreas)
-- Catalin
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